Method and apparatus for forming a tubular billet about a mandrel using multi-directional stress

ABSTRACT

In the manufacture of a tubular member which is to be concave inward as viewed in longitudinal section and precisely symmetrical on opposite sides of its transverse centerline, such as a hi-pass millimeter waveguide filter, a tubular billet initially is fabricated with temporary end flanges having circumferential grooves for receiving hydrostatic seals during a stress-forming operation. Next, the tubular billet is positioned in a hydrostatic forming device and a split mandrel is positioned within the billet and retained against movement. Hydrostatic pressure then is applied to the tubular billet to stress the billet longitudinally in tension while compressively stressing the billet inward into formed engagement with the mandrel, to reduce the hydrostatic pressure required in the stress-forming operation and to achieve the desired symmetry in the formed billet. The hydrostatic stress-forming of the billet preferably is accomplished in successive steps, with intermediate annealing, using mandrels of progressively decreasing size, and the billet may be stretched longitudinally while being formed inward. When the hydrostatic stress-forming of the billet has been completed, portions of the temporary end flanges which define the seal receiving grooves are removed from each end flange and the end flanges are formed to a final desired configuration.

United States Patent [1 1 Shatter i 1 June 17, 1975 METHOD AND APPARATUS FOR FORMING A TUBULAR BILLET ABOUT A MANDREL USING MULTI-DIRECTIONAL STRESS John Richard Shaffer, Ewing Twp., Mercer County, NJ.

[75] Inventor:

[73] Assignee: Western Electric Company,

Incorporated, New York, N.Y.

221 Filed: Mar. 25, 1974 211 App]. No.: 454,648

Primary Examiner-Richard J. Herbst Attorney, Agent, or Firm-D. D. Bosben [57] ABSTRACT In the manufacture of a tubular member which is to be concave inward as viewed in longitudinal section and precisely symmetrical on opposite sides of its transverse centerline, such as a hi-pass millimeter waveguide filter, a tubular billet initially is fabricated with temporary end flanges having circumferential grooves for receiving hydrostatic seals during a stress-forming operation. Next, the tubular billet is positioned in a hydrostatic forming device and a split mandrel is positioned within the billet and retained against movement. Hydrostatic pressure then is applied to the tubular billet to stress the billet longitudinally in tension while compressively stressing the billet inward into formed engagement with the mandrel, to reduce the hydrostatic pressure required in the stress-forming operation and to achieve the desired symmetry in the formed billet. The hydrostatic stress-forming of the billet preferably is accomplished in successive steps, with intermediate annealing, using mandrels of progressively decreasing size, and the billet may be stretched longitudinally while being formed inward. When the hydrostatic stressforming of the billet has been completed, portions of the temporary end flanges which define the seal receiving grooves are removed from each end flange and the end flanges are formed to a final desired configuration.

11 Claims, 7 Drawing Figures PATENTEUJUN 17 I975 SHEET it N) cum 1 METHOD AND APPARATUS FOR FORMING A TUBULAR BILLET ABOUT A MANDREL USING MULTI-DIRECTIONAL STRESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method and apparatus for forming a tubular billet about a mandrel using multidirectional stress, and more particularly to a method and apparatus for forming a hi-pass millimeter wave guide filter which is to have a precise inwardly concave configuration as viewed in longitudinal section and to be precisely symmetrical on opposite sides of its transverse centerline.

2. Description of the Prior Art In a circular millimeter waveguide for transmitting signal information over long distances, it is necessary to interpose an interferometer comprising a system of hipass filters in the waveguide at periodic intervals, to separate the signal information into bands for regeneration and amplification. A proposed form of filter for this purpose, which passes high frequency waves and reflects low frequency waves, is of tubular metal construction and has a precise internal configuration which is concave inward as viewed in longitudinal section, with its contour defined primarily by an exponential curve surface of revolution. It is essential that this filter be fabricated so that the internal surface of the filter is smooth and uninterrupted, and so that the filter is symmetrical on opposite sides of its transverse centerline to avoid extraneous reflections and/or other distortions of the signal information received by the filter.

Further, since the filters are utilized in matched pairs in the interferometer, with each pair of filters forming part of a balanced subsystem or diplexer, it is essential that the matched pairs have identical operating characteristics. This requires that the filters of each pair be manufactured so as to be identical with one another. Otherwise, an unbalance is produced in the interferometer whereby the transmitted and reflected wave frequencies in the diplexer do not phase properly at their respective hybrid couplers, causing degradation in the performance of the interferometer and a reduction in the efficiency and the signal carrying capability of the waveguide.

Prior known techniques commonly used for fabricating waveguide filters, such as machining and then soldering filter sections together, or by electroforming, are not suitable for forming this one-piece waveguide filter to the necessary precision and symmetry. For example, in electroforming, in which a filter is fabricated by eletrodepositioning a suitable metal about a mandrel, since the proposed filter has straight internal end sections and a straight internal central transition portion (on the order of I mils in length), it is difficult to withdraw the mandrel from the electrodeposited filter without marring or otherwise damaging the surface of the mandrel. On the other hand, using disposable mandrels is undesirable because of the expense involved and because it is difficult to machine successive mandrels to the same duplicate configuration with the necessary degree of accuracy.

Electroforming also is disadvantageous because internal stresses produced in the filter during the electrodeposition process cause deformation of the filter when the stresses are relieved by removal of the mandrel. In contrast, a filter produced by cold flow of the metal,

providing the necessary precision and symmetry can be obtained, will become work-hardened during the forming process and be substantially free of internal stress. Merely cold-forming a tubular metal billet inward about a mandrel and obtaining the desired precision and symmetry is difficult, however, because of the distortions created in the billet during the forming process, and/or because of the high forming pressures required to produce cold flow in the metal by overcoming its compressive strength.

Of related interest to this invention is the US. Pat. No. 3,3l4,266, issued Apr. I8, 1967 to O. Werther et al., which discloses a method of forming a tubualr metal billet into a shortened pipe coupling having a cylindrical outer surface and an inwardly concave internal surface, using a split mandrel and longitudinal compressive pressure on the ends of the billet. Again, however, this forming process is not suitable for forming the subject waveguide filter because it requires producing cold flow in a metal billet by utilizing brute force to overcome its compressive strength. Thus, to produce the cold flow in the billet necessary to achieve the precision and symmetry required in the finished waveguide filter would be difficult and would require an abnormally high forming pressure. Also of related interest, but not suitable for forming the subject waveguide filter, is apparatus disclosed in the US. Pat. Nos. 3,358,488, and 3,358,489, for the hydrostatic forming ofa cylindrical tubular metal billet into an outward bulbous configuration.

SUMMARY OF THE INVENTION In accordance with this invention, a tubular billet is formed inwardly about a mandrel having an external surface which corresponds to a desired internal configuration of the billet, by stressing the billet inward in tension while compressively stressing the billet inward about the mandrel.

More specifically, a tubular metal billet initially is formed with temporary end flanges having opposed annular bearing surfaces and circumferential grooves for receiving hydrostatic seals during a stress-forming operation. The tubular billet then is placed in a hydrostatic forming device with the end flanges confined against transverse movement by cylindrical interior surface portions of the forming device, and with the hydrostatic seals in sealing engagement with the surface portions. A split mandrel having an external configuration conforming to a desired internal configuration of the bilet also is positioned therein and retained against movement. Hydrostatic pressure then is applied to the opposed annular bearing surface of the temporary end flanges and the peripheral portion of the tubular billet therebetween to stress the billet longitudinally in tension, while compressively stressing the billet inward into formed engagement with the split mandrel. Preferably, the tubular billet actually is stretched longitudinally and uniformly on opposite sides of its transverse centerline and relative to the mandrel, during the stress-forming operation. Subsequently, portions of the temporary end flanges defining the opposed annular bearing surfaces and the circumferential grooves are removed from the end flanges and the end flanges are formed to a desired final configuration.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of a waveguide filter which may be formed by a method and apparatus in accordance with the invention;

FIG. 2 illustrates certain initial steps in the forming of a tubular billet in accordance with the invention, to produce the waveguide filter shown in FIG. 1;

FIG. 3 is a cross-sectional view of apparatus in accordance with the invention for hydrostatically forming the tubular billet shown in FIG. 2',

FIG. 4 is an enlarged cross-sectional view of a portion of the apparatus shown in FIG. 3;

FIG. 5 is an enlarged cross-sectional view taken along the line 55 in FIG. 3;

FIG. 6 is a erosssectional view of an alternate embodiment of apparatus in accordance with the invention, prior to the hydrostatic forming of a tubular billet as shown in FIG. 2; and

FIG. 7 is a crosssectional view of the apparatus shown in FIG. 6 after the tubular billet as shown in FIG. 2 has been hydrostatically formed in accordance with the invention.

DETAILED DESCRIPTION The disclosed embodiments ofthe invention relate to the fabricating of a tubular waveguide filter ll, shown in FIG. 1, which has internal and external surfaces [11' and 110 of an inwardly concave configuration as viewed in longitudinal section. The filter 11 includes identical end flanges llffor use in subsequently incorporating the filter into a waveguide interferometer (not shown), and is symmetrical on opposite sides of its transverse centerline 11!. as is well known to those skilled in the art. More specifically, the internal contour of the filter 11 is defined by an exponential curve surface of revolution. with straight end sections and a straight central transition section and, as viewed in transverse section at any point along its length, the inner and outer surfaces 111' and 110 of the filter are concentric about its longitudinal axis. As a result of this perfect symmetry. which can be achieved utilizing this invention, and since filters can be repetitively formed utilizing this invention with a high degree of accuracy so that consecutively formed filters are essentially identical, the filters can be utilized in the aforementioned interferometer as matched pairs having substantially identical operating characteristics so as to produce a balanced system having maximum efficiency and transmission capability.

In general, with regard to the embodiment of the invention shown in FIGS. 3-5, and referring to FIG. 2, an extruded tubular sleeve (not shown) of a ductile metal suitable for the waveguide filter 11, such as copper, initially is machined to form a tubular billet 12 having uniform internal and external diameters. The billet 12 also is formed with identical temporary integral cylindrical end flanges 13 having an external diameter greater than the external diameter of the billet between the end flanges and which include identical opposed annular bearing surfaces 13b intersecting the external surface of the billet to provide seals at their respective intersections, circumferential grooves 13g and outer flange rings 130. The tubular billet 12 then is positioned in a hydrostatic forming device 14 as illustrated in FIG. 3, with annular O ring hydrostatic seals 16 positioned in the grooves, as is shown in FIG. 4. In the forming device l4, hydrostatic pressure is applied to the opposed annular bearing surfaces 13b on the end flanges 13 to stress the billet 12 longitudinally in tension, while simultaneously applying the hydrostatic pressure to the exterior of the billet between the end flanges to compressively stress and form the billet inward about a split mandrel consisting of two mandrel halves 17 and 18 having highly polished smooth exterior surfaces. Preferably, this stress-forming of the billet 12 to the final configuration of the waveguide filter 11 is accomplished in a series of successive steps, with full annealing of the billet prior to each step, using mandrels l7, 18 of progressively decreasing size. After the final forming operation. the portions of the temporary end flanges l3 defining the annular bearing surfaces 13b and the seal receiving grooves 13g are removed from the billet 12 by machining and the end flanges are formed to their desired final configuration as illustrated by the end flanges llfin FIG. 1.

The stressing of the billet I2 longitudinally in tension is such as to retain the end flanges 13 against any substantial inward movement during the stress-forming operation, and preferably is sufficient to cause slight stretching of the billet uniformly on opposite sides of its transverse centerline 12t (FIG. 2). This eliminates distortions in the billet 12 as a result of the end flanges 13 tending to be pulled toward one another during the compressive forming of the billet inward about the mandrel I7, 19, thus producing a desired symmetrical configuration in the billet as above described and as illustrated in FIGS. 3 and 5. At the same time, maintaining the billet 12 in longitudinal tension also reduces the hydrostatic pressure required to form the billet compressively inward about the mandrel 17, 18.

In fabricating the temporary end flanges 13 on the tubular metal billet 12, the opposed annular bearing surfaces 13b are formed with a cross-sectional area of sufficient magnitude to insure that during the stress forming of the billet, the hydrostatic pressure will maintain the billet in longitudinal tension as above described, taking into consideration the radial wall thickness and the ductility of the billet. Similarly, the temporary end flanges 13 must be strong enough to avoid failare in shear during the stress forming operation. By way of illustration and referring to FIG. 4, in forming tubular copper billets 12 having a radial wall thickness on the order of 3/32 of an inch, favorable results have been achieved with end flanges 13 having a radial dimension r and a thickness dimension 1 on the order of the wall thicknessv As is best shown in FIG. 4, each circumferential groove 13 also is formed in its respective temporary end flange 13 to provide a hydrostatic sealing arrangement of a type known to those skilled in the art, and including an undercut portion 13a. The undercut portion 13a functions to entrap the associated hydrostatic seal 16 in the circumferential groove 13g during a hydrostatic forming operation, and also tends to cold-flow during the stress-forming operation to facilitate the forming of a metal-to-metal seal between the temporary end flange 13 of the billet l2 and the hydrostatic forming device 14.

As noted above, the stress-forming of the tubular metal billet 12 to the inwardly formed configuration of the waveguide filter 11 shown in FIG. 1 preferably is accomplished in a series of hydrostatic forming steps, with the mandrel 17, 18 used in each step progressively decreasing in size, that is, of progressively smaller diameters intermediate their ends, the diameters of which ends remain constant. By way of illustration, the

cross-sectional area of the billet 12 at its transverse centerline 121 may be reduced in successive steps in a range on the order of 22 to 33% in each step. This facilitates symmetrical forming of the billet 12, and with annealing of the billet prior to each step (e.g., at l,l00F. for 1 /2 hours in a nitrogen atmosphere, for copper), also reduces the hydrostatic forming pressure required as compared to that necessary to form the billet in a single step, thus reducing the possibility of breaking the mandrel 17, 18 during the forming operation.

In each of the successive hydrostatic forming steps, the final one of which is depicted in FIG. 3, the tubular billet 12 is positioned in a cylindrical forming chamber 19f extending through a housing 19 of the hydrostatic forming device 14, with the temporary end flanges 13 of the billet received in close-fitting relationship with respect to smooth cylindrical interior surfaces 19s of the housing, and thus confined against transverse movement with respect to the longitudinal axis of the billet, as a result of the flanges having been formed with an external diameter substantially corresponding to the diameter of the cylindrical interior surfaces. The annular hydrostatic seals 16, which have been positioned in the circumferential grooves 133 in the temporary end flanges 13, also are located in sealing engagement with the interior cylindrical surfaces 19s, as illustrated in FIG. 4. To facilitate the insertion of the billet 12 in the hydrostatic forming device 14, and withdrawal of the billet therefrom, the cylindrical forming chamber 19f may be diametrically enlarged relative to the cylindrical interior surfaces 19s, as shown in FIGS. 3 and 4.

The split mandrel 17, 18, having an external surface which corresponds to the desired internal configuration of the billet 12, also is positioned within the billet, with inner opposed ends of the mandrel halves 17 and 18 aligned by a suitable pilot pin 21 disposed in opposed passageways therein. A pair of metal plugs 22 and 23 are adapted to be screw threaded into the housing 19 of the hydrostatic forming device 14 into tight engagement with respective opposite ends of the mandrel halves 17 and 18 to force their inner ends into tight engagement and to retain them against movement during the hydrostatic stress-forming operation. Cylindrical metal spacers 24 and 26 of precise thickness also are slidably inserted between the metal plugs 22, 23 and the opposite ends of the tubular billet 12, with the spacers preferably dimensioned so a slight gap (such as several thousandths of an inch) exists between the spacers and the ends of the billet, to permit a slight stretching of the billet longitudinally during the forming operation.

With the billet l2 assembled in the hydrostatic forming device 14 of FIG. 3, a suitable liquid 27, such as hydraulic oil, is poured into a pressurizing chamber 19p of the housing 19 and the cylindrical forming chamber 19f. A hydraulic ram 28 then is positioned in the pressurizing chamber 19p and moved downward therein in a known manner to pressurize the liquid 27. The resul tant hydrostatic pressure, which for copper may be on the order of 75,000 psi, is exerted upon the opposed annular bearing surfaces 13b of the temporary end flanges 13 of the billet l2 (and upon the hydrostatic seals 16) to stress the billet longitudinally in tension and to stretch the billet uniformly in opposite directions into firm engagement with the spacers 24 and 26, thereby forcing the spacers against the metal plugs 22 and 23 to positively limit the stretching of the billet and thus establish a precise length for the formed billet. At the same time, the hydrostatic pressure is exerted upon the exterior of the billet 12 between the end flanges 13 to compressively stress the billet inward into formed engagement with the split mandrel 17, 18. Preferably, the hydrostatic pressure is applied to the billet 12, relieved, and reapplied and relieved again one or more times, with final relief being accomplished slowly by reducing the pressure in progressive stages.

Referring to FIG. 4, in the hydrostatic forming of the tubular billet 12, each annular seal 16 initially precludes flow of the liquid 27 past its temporary end flange 13. Subsequently, as the metal of the tubular billet 12 cold-flows as a result of the stress exerted thereon by the liquid 27, the outermost portion of the end flange 13 flows into tight engagement with the adjacent interior surface 19s of the housing 19 to form a metal-to-metal seal between the end flange and the housing. The formation of this metal-to-metal seal is enhanced as a result of the undercut portion 131: of the end flange l3 entrapping the annular seal 16, with the seal and the hydrostatic liquid then exerting forces on the undercut portion in a cantilever fashion to cause it to move outward into engagement with the interior surface l9s, in a manner known to those skilled in the art.

During the forming of the billet 12 about the mandrel 17, 18, air between the interior of the billet and the mandrel is vented therefrom through a small passage 17v in the mandrel half 17, the passage extending axially along the centerline of the mandrel half, opening through the surface of the mandrel half adjacent its inner end, and being in communication with a venting passage 22v extending centrally through the metal plug 22 adjacent the outer end of the mandrel half. However, in certain instances, such as in a final forming step in which the mandrel 17, 18 is small in size, to avoid weakening of the mandrel half 17 and causing breakage thereof during the forming operation, and/or to avoid the formation ofa minute protrusion on the inerior surface of the filter 11 by material flowing into the venting passage, it may be desirable to eliminate the venting passage entirely.

After the stress-forming of the billet 12 has been completed, the billet and the mandrel 17, 18 are removed from the hydrostatic forming device 14, the liquid 27 is drained from the device, and the mandrel halves are removed from their respective ends of the billet. For this purpose, as is illustrated by the mandrel half 17, one or both of the mandrel halves 17 and 18 may be internally threaded to provide a recess 17r into which a suitably threaded pulling tool (not shown) can be inserted.

The alternate embodiment of the invention disclosed in FIGS. 6 and 7 is directed to a hydrostatic forming device 14 which is particularly suited for the fabricating of a tubular billet 12 made of a metal of lower ductility than copper and having higher compressive strength or resistance to cold flow. In the apparatus 14', the tubu lar billet 12 actually is elongated or stretched a significant amount (e.g., 20-40%) uniformly in opposite directions during the stress-forming of the billet, thereby significantly reducing the hydrostatic pressure required to compressively form the billet inward into engagement with a split mandrel formed of first and second mandrel parts 17' and 18', as compared to the reduction of pressure achieved with the hydrostatic forming device 14 of FIGS. 3-5. In this regard, stretching of the billet 12' during inward compressive stress-forming thereof is desirable in order to reduce the hydrostatic forming pressure required to a practical operating value, and in order to attain a finished waveguide filter (not shown) of the desired symmetrical cross-section and essentially free of distortions. As in the embodi ment of the invention in FIGS. 3, 4 and 5, the forming of the billet 12' preferably is accomplished in successive stages, with intermediate annealing, using split mandrels 17', 18' of progressively decreasing size.

In utilizing the apparatus 14' of FIGS. 6 and 7, the tubular billet 12 is preformed with temporary end flanges 13' having opposed annular bearing surfaces 13b and hydrostatic seal'receiving circumferential grooves 13g in the same manner as disclosed in the embodiment of the invention shown in FIGS. 3, 4 and 5. However, to avoid failure of the temporary end flanges 13' in shear, it may be necessary to make them of an initial thickness (1 in FIG. 4) considerably greater (e.g., of an inch) than that required in the hydrostatic forming device 14 of FIGS. 3, 4 and 5, and then progressively reduce their thickness in each successive forming step to achieve the desired results. The billet 12' then is positioned in a cylindrical forming chamber 19] ofa main housing 19', as shown in FIG. 6, with the temporary end flanges 13' in close-fitting relationship with cylindrical interior surfaces 19s of the housing, and with annular O-ring hydrostatic seals (not shown) in the grooves 133' in sealing engagement with the interior surfaces. While for purposes of illustration the forming ofthe tubular billet 12' is shown in FIGS. 6 and 7 as being accomplished in a single step, as discussed above with respect to the embodiment of FIGS. 3, 4 and 5, and as noted in the preceding paragraph, the stress-forming of the billet preferably is accomplished in a series of successive steps with mandrels 17', 18' of progressively decreasing size, with intermediate annealing, to reduce the hydrostatic forming pressure required and to avoid mandrel breakage.

In the apparatus 14', the ends of the billet 12' bear against respective sleeve members 31 and 32 each having a portion slidably mounted in the adjacent end of the cylindrical forming chamber 19f and in :1 respec tive guide bushing 33 or 34 mounted in an adjacent subhousing 36 or 37. Coil springs 38 and 39 of equal yieldable strength, so as to produce equal biasing and centering forces on the billet 12', are disposed about portions of the sleeve members 31 and 32, between in termediate collars integral therewitth and respective interior walls 361 and 371' of the subhousings 36 and 37.

The first mandrel part 17' is long relative to the second mandrel part 18' and extends through its respective sleeve member 31 into butting engagement with the interior wall 361' of the subhousing 36. The long mandrel part 17' also includes a central longitudinally extending venting passage 17v which at its inner end opens through the periphery of the mandrel part to evacuate air from between the mandrel and the billet 12' during a forming operation, and which at its outer end communicates with an aligned venting passage 36v in the subhousing 36.

The short mandrel part 18 extends into one end of the adjacent sleeve member 32, in which it is butted against a cylindrical mandrel retaining member 41. The mandrel retaining member 41 projects out of the other end of the sleeve member 32 and through a guide bushing 42 fixedly mounted in the subhousing 37. Secured to the projecting outer end of the mandrel retaining member 41 is a piston head 43, which is mounted in a small hydraulic chamber 3711 in the subhousing 37 and provided with a suitable O-ring hydrostatic seal. The hydraulic chamber 371: communicates with a hydro static pressurizing chamber 19p in the main housing 19 via an upwardly extending passageway 37p drilled in the subhousing 37, a central passage through a high pressure tubing member 44, and a passageway 46 drilled in the main housing.

The high pressure tubing member 44 is disposed in aligned recesses in the main housing 19 and the subhousing 37 so as to preclude leakage of hydrostatic liquid 27 between the mating faces of the housing and the subhousing during a stress-forming operation. In this regard, adjacent its right-hand end as viewed in FIG. 6, the tubing member 44 is screw threadably mounted in the main housing 19' and is provided with a suitable hydrostatic seal, and adjacent its left-hand end in this figure the tubing member is provided with another suitable hydrostatic seal held in place by a screw threaded retaining nut 47. The tubing member 44 also precludes passage of the hydrostatic liquid 27' out of an upper extension of the communication passage 37;) in the subhousing 37, the upper extension being inherently formed in the subhousing in connection with the drilling of the lower portion of the communication passage therein.

With the tubular billet 12 assembled in the apparatus 14' as shown in FIG. 6, a hydraulic ram 28' is moved downward in the pressurizing chamber 19p to place the liquid 27' under pressure. This causes hydrostatic pressure to be exerted on the opposed annular bearing surfaces 13b of the temporary end flanges 13' to cause an actual stretching or lengthening of the billet 12' against the biasing and centering action of the coil springs 38 and 39. At the same time, the hydrostatic pressure is exerted on the exterior of the billet 12' between the temporary end flanges 13' to compressively form the billet inward into engagement with the split mandrel 17', 18. During the stress-forming of the billet 12', a portion of the hydrostatic liquid 27' in the pres sure chamber 19' also is diverted through the passageway 46 in the housing 19, the high pressure tubing member 44 and the passageway 37p in the subhousing 37, to exert hydrostatic pressure on the piston head 43 of the mandrel retaining member 41, so as to hold the mandrel parts 17' and 18' tightly in proper position during the forming operation. Thus, any movement of the mandrel parts 17' and 18' due to tolerance allowances in constructing the apparatus 14', as might be the case where both mandrel parts were butted against their respective subhousing interior walls 361' and 371', is eliminated.

In utilizing the apparatus 14', the billet 12 preferably is stretched by hydrostatic pressure until the sleeve members 31 and 32 have been moved outward into engagement with the interior wall 361' of the subhousing and the bushing 42 in the subhousing 37, respectively, as illustrated in FIG. 7. Thus, the sleeve members 31 and 32 can function in the same manner as the spacers 24 and 26 in the embodiment of the invention in FIGS. 3, 4 and 5, to positively limit the elongation of the billet l2 and produce a formed billet of precise length.

As in the embodiment of the invention in FIGS. 3, 4 and 5, after the stress-forming of the billet 12' has been completed, the portions of the end flanges 13' defining the opposed annular bearing surfaces 13b and the sealreceiving grooves 13g are removed by machining and theendflanges are formed to a final desired configuration as illustrated by the end flanges 11]" of the waveguide filter 11 in FIG. 1.

In summary, an inexpensive method and apparatus has been provided for accurately forming a tubular member, such as the hi-pass waveguide filter 11, so that it is concave inward as viewed in longitudinal section and precisely symmetrical on opposite sides of its trans verse centerline, with a smooth, uninterrupted internal surface contour. Utilizing the disclosed method and apparatus, the filters 11 also can be repetitively formed to a high degree of accuracy with the same mandrel so as to be essentially identical, thus enabling their use as matched pairs in a balanced subsystem or diplexer of a millimeter waveguide interferometer without causing degradation in the performance of the interferometer and a reduction in the efficiency and signal carrying capability of the waveguide.

More specifically, with reference to FIGS. 1-5 by way of example, this is accomplished by stressing the billet 12 longitudinally in tension while compressively stressing the billet inward about the mandrel 17, 18. For this purpose, the temporary end flanges 13 are initially formed on the billet 12 with the opposed annular bearing surfaces 13b and the hydrostatic seal-receiving grooves 13g. This enables the stress-forming of the billet 12 to be accomplished utilizing hydrostatic pressure, which is advantageous because of the uniform forces which are inherently applied to the billet in both compression and tension to accomplish the forming thereof. The stressing of the billet 12 longitudinally in tension during the forming operation is desirable since it reduces the hydrostatic forming pressure required to achieve compressive forming of the billet inward, and eliminates distortions in the billet as a result of the end flanges 13 being pulled toward one another during the inward forming thereof. Similarly, stretching the billet l2 slightly into firm engagement with the spacers 24 and 26 during the forming operation produces a formed billet of precise length. As a net result, the billet 12 readily can be fabricated to the desired precision and symmetry which is essential in the waveguide filter 11. It also has been found that the mandrel 17, 18 readily can be removed from the formed billet 12 without marring the surface of the mandrel parts, thus enabling reuse of the mandrel and the forming of filters 11 which are of essentially identical configuration.

In addition, the embodiment of the invention shown in FIGS. 6 and 7, in which the billet 12 actually is stretched longitudinally a significant amount during the compressive forming thereof, is particularly advantageous where the billet is of a metal of lower ductility than copper and having higher compressive stress or resistance to cold flow. Specifically, stretching the billet 12' while compressively forming the billet inward is desirable in order for the hydrostatic forming pressure required to compressively form the billet to be reduced to a practical operating value, and in order to eliminate distortions in the billet as a result of the compressive forming thereof, so as to achieve the desired precision and symmetry required in the waveguide filter 11. During the stretching of the billet 12, this achievement of the desired precision and symmetry is facilitated by the equal force centering and biasing springs 38 and 39, the holding of the mandrel parts 17' and 18 tightly against any movement by the hydrostatic pressure on the piston head 43 of the mandrel retaining member 41, and the movable sleeves or spacers 31 and 32, which can be engaged against the inner wall 361' of the subhousing 36 and the bushing 42 in the subhousing 37 to produce a formed billet of precise length.

What is claimed is: l. A method of forming a tubular metal member to a configuration in which internal and external surfaces of the member are concave inward as viewed in longitudinal section and at least the internal surfaces are symmetrical on opposite sides of the transverse centerline of the member, which comprises:

forming a tubular metal billet having an external surface of uniform diameter and an internal surface of uniform diameter; forming the tubular metal billet with substantially identical temporary integral end flanges having an external diameter greater than the external diameter of the billet between the end flanges and substantially corresponding to a diameter of cylindrical interior surfaces of a hydrostatic forming device, and having opposed annular bearing surfaces intersecting the external surface of the billet to provide hydrostatic seals at their respective intersections and of sufficient cross-sectional area such that when the bearing surfaces and the peripheral portion of the billet therebetween subsequently are subjected to hydrostatic pressure in a forming operation, the end flanges are retained against any substantial inward longitudinal movement and the billet is stressed longitudinally in tension; forming each temporary integral end flange of the tubular billet with a circumferential seal-receiving portion; positioning annular hydrostatic seals in engagement with the circumferential seal-receiving portions; positioning the tubular billet in the hydrostatic forming device with the temporary integral end flanges confined against transverse movement by the cylindrical interior surfaces of the forming device, and with the seals in sealing engagement with the cylindrical interior surfaces; positioning a split mandrel having an external configuration conforming to the desired internal concave configuration of the tubular member, within the tubular billet with opposed inner ends of the mandrel in aligned engagement and spaced relationship to the interior of the billet; subjecting the opposite outer ends of the split mandrel to opposing forces to preclude longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the forming of the billet inwardly about the mandrel; simultaneously applying hydrostatic pressure to the opposed annular bearing surfaces of the temporary integral end flanges and to the peripheral portion of the tubular billet therebetween, to retain the end flanges against any substantial inward longitudinal movement and to stress the billet longitudinally in tension, while compressively stressing the billet inward into inwardly concave symmetrical engagement with the mandrel; and removing selected portions of the temporary integral end flanges not required in the tubular metal member after the stress-forming of the billet. 2. A method as recited in claim 1, wherein:

the forming of the tubular billet is accomplished in a series of separate hydrostatic forming steps utilizing mandrels of progressively decreasing size; and

the tubular billet is annealed between each hydro static forming step.

3. A method as recited in claim I, in which:

the portions of the temporary integral end flanges which are removed after the stress-forming of the billet include the opposed annular bearing surfaces and the seal receiving portions.

4. A method as recited in claim 1, in which:

the seal-receiving portion in each temporary integral end flange is formed as a groove with an undercut seal-entrapping portion; and

the portions of each end flange which are removed after the stressforming of the billet include the groove and the undercut entrapping portion.

5. A method as recited in claim 1, which further comprises:

applying the hydrostatic pressure to the billet so as to stretch the billet longitudinally and uniformly on opposite sides of its transverse centerline relative to the mandrel while compressively stressing the billet inward about the mandrel; and

positively limiting the stretching of the billet longitudinally relative to the mandrel to produce a formed billet of precise length.

6. A method as recited in claim 5, which further comprises:

exerting equal yieldable biasing and centering forces on opposite ends of the tubular billet in the hydrostatic forming device; and

applying the hydrostatic pressure to the opposed annular bearing surfaces of the end flanges to stretch the tubular billet longitudinally and uniformly on opposite sides of its transverse centerline against the yieldable biasing action of the centering forces and relative to the mandrel and the interior cylindrical surfaces of the hydrostatic forming device, while simultaneously applying the hydrostatic pressure to the peripheral portion of the tubular billet between the bearing surfaces to compressively stress the billet inward into formed engagement with the mandrel.

7. A method as recited in claim 6, which further comprises:

diverting a portion of a hydrostatic liquid for applying the hydrostatic pressure to the tubular billet, to produce one of the forces exerted on one of the outer ends of the split mandrel to retain the split mandrel in a desired position against separation and longitudinal movement during the stressforming of the billet.

8. A hydrostatic method of forming a tubular metal member to a configuration in which internal and external surfaces of the member are concave inward as viewed in longitudinal section and at least the internal surfaces are symmetrical on opposite sides of the transverse centerline of the member, which comprises:

forming a tubular metal billet having an external surface of uniform diameter and an internal surface of uniform diameter;

positioning the tubular billet and a hydrostatic forming device with both of the opposite ends of the billet capable of longitudinal movement but confined against transverse movement;

ltl

positioning a split mandrel having an external surface which is concave inward as viewed in longitudinal section and which corresponds to a desired internal configuration of the metal member, within the billet with opposed inner ends of the mandrel in aligned engagement and in spaced relationship to the interior of the billet;

subjecting the opposite outer ends of the split mandrel to opposing forces to preclude longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the hydrostatic forming of the billet inwardly about the mandrel;

exerting equal yieldable biasing and centering forces on opposite ends of the billet; and

simultaneously applying hydrostatic pressure to 0pposed annular bearing surfaces on opposite ends of the billet and to the peripheral portion of the billet between the bearing surfaces to stretch the billet longitudinally against the yieldable biasing action of the centering forces and uniformly on opposite sides of the transverse centerline of the billet relative to the split mandrel, while compressively forming the billet inward into inwardly concave symmetrical engagement with the split mandrel.

9. A method as recited in claim 8, which further comprises:

diverting a portion of a hydrostatic liquid for applying the hydrostatic pressure to the tubular billet to produce one of the forces exerted on one of the outer ends of the split mandrel to retain the split mandrel against separation and longitudinal movement during the stress-forming of the billet.

l0. Hydrostatic apparatus for forming a tubular metal billet having end flanges and of uniform internal and external diameter between the end flanges, into a member having internal and external surfaces which are concave inward as viewed in longitudinal section, with at least the internal surfaces being symmetrical in configuration on opposite sides of the transverse centerline of the member, which comprises:

a housing having a cylindrical hydrostatic forming chamber for receiving the billet and including cylindrical interior surface portions engageable with the end flanges on the billet to permit longitudinal movement of both end flanges while confining the end flanges against transverse movement;

a split mandrel having an inwardly concave external configuration which corresponds to the desired internal configuration of the billet, said mandrel being positionable in the billet with inner opposed ends thereof in aligned engagement and spaced relationship to the interior of the billet;

retaining means engageable with the opposite outer ends of the split mandrel for precluding longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the forming of the billet;

a pair of rigid cylindrical members slidably positionable on respective parts of the split mandrel and engageable with opposite ends of the billet;

means engageable by said rigid cylindrical members for positively limiting stretching of the billet so as to produce a formed billet of precise length;

first and second resilient means between said limiting means and said cylindrical members for exerting equal, yieldable biasing and centering forces on said cylindrical members and thus on the opposite ends of the billet; and

means for applying hydrostatic pressure to the end flanges on the billet to stretch the billet uniformly on opposite sides of its transverse centerline relative to the split mandrel and the interior surfaces of the cylindrical chamber in said housing and to move said slidable members on said mandrel against the biasing and centering action of said first and second resilient means into engagement with said limiting means, while simultaneously applying hydrostatic pressure to the exterior of the billet between the end flanges to compressively stress the billet into inwardly concave symmetrical engagement with the split mandrel. 11. Hydrostatic apparatus for forming a tubular metal billet, as recited in claim 10, in which:

ing the stress-forming of the billet. 

1. A method of forming a tubular metal member to a configuration in which internal and external surfaces of the member are concave inward as viewed in longitudinal section and at least the internal surfaces are symmetrical on opposite sides of the transverse centerline of the member, which comprises: forming a tubular metal billet having an external surface of uniform diameter and an internal surface of uniform diameter; forming the tubular metal billet with substantially identical temporary integral end flanges having an external diameter greater than the external diameter of the billet between the end flanges and substantially corresponding to a diameter of cylindrical interior surfaces of a hydrostatic forming device, and having opposed annular bearing surfaces intersecting the external surface of the billet to provide hydrostatic seals at their respective intersections and of sufficient crosssectional area such that when the bearing surfaces and the peripheral portion of the billet therebetween subsequently are subjected to hydrostatic pressure in a forming operation, the end flanges are retained against any substantial inward longitudinal movement and the billet is stressed longitudinally in tension; forming each temporary integral end flange of the tubular billet with a circumferential seal-receiving portion; positioning annular hydroStatic seals in engagement with the circumferential seal-receiving portions; positioning the tubular billet in the hydrostatic forming device with the temporary integral end flanges confined against transverse movement by the cylindrical interior surfaces of the forming device, and with the seals in sealing engagement with the cylindrical interior surfaces; positioning a split mandrel having an external configuration conforming to the desired internal concave configuration of the tubular member, within the tubular billet with opposed inner ends of the mandrel in aligned engagement and spaced relationship to the interior of the billet; subjecting the opposite outer ends of the split mandrel to opposing forces to preclude longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the forming of the billet inwardly about the mandrel; simultaneously applying hydrostatic pressure to the opposed annular bearing surfaces of the temporary integral end flanges and to the peripheral portion of the tubular billet therebetween, to retain the end flanges against any substantial inward longitudinal movement and to stress the billet longitudinally in tension, while compressively stressing the billet inward into inwardly concave symmetrical engagement with the mandrel; and removing selected portions of the temporary integral end flanges not required in the tubular metal member after the stressforming of the billet.
 2. A method as recited in claim 1, wherein: the forming of the tubular billet is accomplished in a series of separate hydrostatic forming steps utilizing mandrels of progressively decreasing size; and the tubular billet is annealed between each hydrostatic forming step.
 3. A method as recited in claim 1, in which: the portions of the temporary integral end flanges which are removed after the stress-forming of the billet include the opposed annular bearing surfaces and the seal receiving portions.
 4. A method as recited in claim 1, in which: the seal-receiving portion in each temporary integral end flange is formed as a groove with an undercut seal-entrapping portion; and the portions of each end flange which are removed after the stress-forming of the billet include the groove and the undercut entrapping portion.
 5. A method as recited in claim 1, which further comprises: applying the hydrostatic pressure to the billet so as to stretch the billet longitudinally and uniformly on opposite sides of its transverse centerline relative to the mandrel while compressively stressing the billet inward about the mandrel; and positively limiting the stretching of the billet longitudinally relative to the mandrel to produce a formed billet of precise length.
 6. A method as recited in claim 5, which further comprises: exerting equal yieldable biasing and centering forces on opposite ends of the tubular billet in the hydrostatic forming device; and applying the hydrostatic pressure to the opposed annular bearing surfaces of the end flanges to stretch the tubular billet longitudinally and uniformly on opposite sides of its transverse centerline against the yieldable biasing action of the centering forces and relative to the mandrel and the interior cylindrical surfaces of the hydrostatic forming device, while simultaneously applying the hydrostatic pressure to the peripheral portion of the tubular billet between the bearing surfaces to compressively stress the billet inward into formed engagement with the mandrel.
 7. A method as recited in claim 6, which further comprises: diverting a portion of a hydrostatic liquid for applying the hydrostatic pressure to the tubular billet, to produce one of the forces exerted on one of the outer ends of the split mandrel to retain the split mandrel in a desired position against separation and longitudinal movement during the stress-forming of the billet.
 8. A hydrostatic method of forming a tubulAr metal member to a configuration in which internal and external surfaces of the member are concave inward as viewed in longitudinal section and at least the internal surfaces are symmetrical on opposite sides of the transverse centerline of the member, which comprises: forming a tubular metal billet having an external surface of uniform diameter and an internal surface of uniform diameter; positioning the tubular billet and a hydrostatic forming device with both of the opposite ends of the billet capable of longitudinal movement but confined against transverse movement; positioning a split mandrel having an external surface which is concave inward as viewed in longitudinal section and which corresponds to a desired internal configuration of the metal member, within the billet with opposed inner ends of the mandrel in aligned engagement and in spaced relationship to the interior of the billet; subjecting the opposite outer ends of the split mandrel to opposing forces to preclude longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the hydrostatic forming of the billet inwardly about the mandrel; exerting equal yieldable biasing and centering forces on opposite ends of the billet; and simultaneously applying hydrostatic pressure to opposed annular bearing surfaces on opposite ends of the billet and to the peripheral portion of the billet between the bearing surfaces to stretch the billet longitudinally against the yieldable biasing action of the centering forces and uniformly on opposite sides of the transverse centerline of the billet relative to the split mandrel, while compressively forming the billet inward into inwardly concave symmetrical engagement with the split mandrel.
 9. A method as recited in claim 8, which further comprises: diverting a portion of a hydrostatic liquid for applying the hydrostatic pressure to the tubular billet to produce one of the forces exerted on one of the outer ends of the split mandrel to retain the split mandrel against separation and longitudinal movement during the stress-forming of the billet.
 10. Hydrostatic apparatus for forming a tubular metal billet having end flanges and of uniform internal and external diameter between the end flanges, into a member having internal and external surfaces which are concave inward as viewed in longitudinal section, with at least the internal surfaces being symmetrical in configuration on opposite sides of the transverse centerline of the member, which comprises: a housing having a cylindrical hydrostatic forming chamber for receiving the billet and including cylindrical interior surface portions engageable with the end flanges on the billet to permit longitudinal movement of both end flanges while confining the end flanges against transverse movement; a split mandrel having an inwardly concave external configuration which corresponds to the desired internal configuration of the billet, said mandrel being positionable in the billet with inner opposed ends thereof in aligned engagement and spaced relationship to the interior of the billet; retaining means engageable with the opposite outer ends of the split mandrel for precluding longitudinal movement of the mandrel and separation of the engaged inner ends of the mandrel during the forming of the billet; a pair of rigid cylindrical members slidably positionable on respective parts of the split mandrel and engageable with opposite ends of the billet; means engageable by said rigid cylindrical members for positively limiting stretching of the billet so as to produce a formed billet of precise length; first and second resilient means between said limiting means and said cylindrical members for exerting equal, yieldable biasing and centering forces on said cylindrical members and thus on the opposite ends of the billet; and means for applying hydrostatic pressure to the end flanges on the billet to stretch the billet uniFormly on opposite sides of its transverse centerline relative to the split mandrel and the interior surfaces of the cylindrical chamber in said housing and to move said slidable members on said mandrel against the biasing and centering action of said first and second resilient means into engagement with said limiting means, while simultaneously applying hydrostatic pressure to the exterior of the billet between the end flanges to compressively stress the billet into inwardly concave symmetrical engagement with the split mandrel.
 11. Hydrostatic apparatus for forming a tubular metal billet, as recited in claim 10, in which: said mandrel retaining means includes at least one slidably-mounted mandrel retaining member; and said housing includes a hydrostatically sealed passageway for directing a portion of a hydrostatic liquid which is pressurized by said hydrostatic pressure applying means, against said slidable mandrel retaining member to retain the split mandrel against separation and longitudinal movement during the stress-forming of the billet. 